ipv4: use RCU protection in ip_dst_mtu_maybe_forward()

ip_dst_mtu_maybe_forward() must use RCU protection to make
sure the net structure it reads does not disappear.

Fixes: f87c10a ("ipv4: introduce ip_dst_mtu_maybe_forward and protect forwarding path against pmtu spoofing")
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

include/net/ip.h

@@ -471,9 +471,12 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 						    bool forwarding)
 {
 	const struct rtable *rt = dst_rtable(dst);
-	struct net *net = dev_net(dst->dev);
-	unsigned int mtu;
+	unsigned int mtu, res;
+	struct net *net;
+
+	rcu_read_lock();
 
+	net = dev_net_rcu(dst->dev);
 	if (READ_ONCE(net->ipv4.sysctl_ip_fwd_use_pmtu) ||
 	    ip_mtu_locked(dst) ||
 	    !forwarding) {
@@ -497,7 +500,11 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 out:
 	mtu = min_t(unsigned int, mtu, IP_MAX_MTU);
 
-	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
+	res = mtu - lwtunnel_headroom(dst->lwtstate, mtu);
+
+	rcu_read_unlock();
+
+	return res;
 }
 
 static inline unsigned int ip_skb_dst_mtu(struct sock *sk,